Thick film dielectric electroluminescent displays are typically fabricated on ceramic, glass ceramic, glass or other heat resistant substrate and provide superior resistance to dielectric breakdown, as well as a reduced operating voltage compared to thin film electroluminescent (TFEL) displays fabricated on glass substrates. The fabrication process for the display entails first depositing a set of row electrodes on the substrate. Next a thick film dielectric layer is deposited, following this, a thin film structure is deposited that comprises one or more thin film dielectric layers sandwiching one or more thin phosphor films and a set of optically transparent column electrodes. The entire structure is covered with a sealing layer that protects the thick and thin film structures from degradation due to moisture or other atmospheric contaminants.
The composite thick film dielectric layers used in such displays have a high dielectric constant, allowing the use of relatively thick dielectric layers in the displays without a significant increase in the display operating voltage. Because the dielectric breakdown strength of these materials is relatively low, relatively thick dielectric layers are used, typically greater than about 10 micrometers, to prevent dielectric breakdown during display operation. Typically, the thick film layer comprises a sintered perovskite piezoelectric or ferroelectric material such as lead magnesium niobate (PMN) or lead magnesium titanate-zirconate (PMN-PT) with a dielectric constant of several thousand. There may also be a thinner overlayer of a compatible piezoelectric or ferroelectric material such as lead zirconate titanate (PZT) applied using metal organic deposition (MOD) or sol gel techniques to smooth the thick film surface for deposition of a thin film phosphor structure.
The Applicant's U.S. Pat. No. 5,432,015 (the disclosure of which is incorporated herein by reference in its entirety) discloses thick film dielectric composite structures for use in electroluminescent displays. The thick film layer is sintered at high temperatures onto a suitable substrate onto which thin film gold electrodes have been applied to achieve a sintered thick film density that is sufficiently high that the remaining porosity, particularly in the upper portion of the layer can be filled by an overlayer deposited using sol gel or MOD techniques. However, the overlayer does not completely fill the porosity of the sintered material since it undergoes a severe volume reduction when the sol gel or MOD precursor materials are fired to form the piezoelectric or ferroelectric material.
The Applicant's PCT patent application Serial No. WO00/70917 (the disclosure of which is incorporated herein by reference in its entirety, discloses an isostatic pressing process by which a deposited thick film dielectric material is mechanically compressed using an isostatic pressing process prior to sintering. This serves the function of increasing the density and decreasing the porosity of the thick film material so that when the overlayer is applied, both the dielectric constant and the dielectric strength of the layer are increased. Dielectric breakdown is associated with random defects in the dielectric layers and the probability of breakdown increases with increasing display area. Thus layers with a higher nominal dielectric strength are desired for use for larger area displays to counteract this tendency.
The Applicant's International Patent Application PCT CA02/01932 (the disclosure of which is incorporated herein by reference in its entirety) discloses a modified thick film paste formulation used to make a thick film dielectric layer. This modified thick film dielectric layer may be sintered at temperatures as low as 650° C. to facilitate the use of a glass substrate. However, the modified thick film dielectric layer still contains residual porosity.
It is therefore desired to provide an improved composite thick film dielectric structure that exhibits fewer defects (i.e. pores or holes) which potentially act as sites for dielectric breakdown and provide a conduit for undesirable reactions between the thick film dielectric layer and phosphor layers leading to reduced display life.